The present invention relates to a self-recovering current-limiting device with liquid metal, including electrodes made of solid metal for the connection to an electric circuit to be protected and a plurality of compression spaces which are partially filled with liquid metal.
Soviet Union Patent Publication SU 922 911 A describes a self-recovering current-limiting device containing electrodes made of solid metal which are separated by first insulating bodies which are designed as a pressure-resistant insulating housing. Inside the insulating housing, compression spaces are formed by insulating intermediate walls and second insulating bodies which are arranged therebetween and designed as ring-shaped sealing disks, the compression spaces being partially filled with liquid metal and arranged one behind the other and interconnected via connecting channels of the intermediate walls, the connecting channels being filled with liquid metal and arranged off-center. Thus, in normal operation, a continuous, inner conductive connection exists between the electrodes via the liquid metal. In the current-limiting event, the liquid metal is displaced from the connecting channels as a result of the high current density. In this manner, the electrical connection of the electrodes via the liquid metal is interrupted, resulting in the limiting of the short-circuit current. Subsequent to clearing or eliminating the short circuit, the connecting channels refill with liquid metal whereupon the current-limiting device is operational again. In German Patent Application DE 40 12 385 A1 , a current-limiting device having only one compression space is described and vacuum, protective gas, or an insulating liquid are mentioned as the medium above the liquid level. According to Soviet Union Patent Publication SU 1 076 981 A, the connecting channels of adjacent intermediate walls are staggered relative to each other for improving the limiting characteristics. In Soviet Union Patent Publication SU 1 094 088 A, copper is specified as a highly conductive material for the electrodes. It is known from German Patent Application DE 26 52 506 A1 to use gallium alloys, in particular GaInSn alloys in contact devices.
During current-limiting events, the inner electrode surfaces come into contact with the developing electric arcs, which in first place gives rise to arc erosion phenomena in the parts of the electrode surfaces that are located opposite of the connecting channels and, secondly, to the contamination of the liquid metal and ultimately, therefore, to an unsatisfactory service life of the current-limiting device. Besides, the current-limiting behavior still deserves improvement.
Therefore, an object of the present invention is to provide a current-limiting device having improved service life and current-limiting behavior.
The present invention provides a self-recovering current-limiting device. The device includes a first and a second electrode for connection to an electric circuit to be protected, each of the first and second electrodes being made of a respective solid metal. A respective non-conductive ceramic disk is disposed at a respective inner surface of each of the first and second electrodes. A plurality of pressure-resistant insulating bodies is provided, and a plurality of insulating intermediate walls are supported by the plurality of insulating bodies. The plurality of insulating intermediate walls and the plurality of pressure-resistant insulating bodies define a plurality of compression spaces, the plurality of compression spaces being disposed one behind the other between the first and second electrodes and being at least partially filled with a liquid metal. The plurality of insulating intermediate walls define a plurality of connecting channels therein, each of the respective non-conductive ceramic disks being disposed opposite of the respective connecting channel of a respective adjacent one of the plurality of insulating intermediate walls.
Because of the non-conductive ceramic disks which locally protect the electrodes, an electric arc developing during external short circuits no longer burns on the electrodes but substantially only in the liquid metal due to the relatively large distance from the conductive regions of the inner surfaces of the electrodes. In this manner, arc erosion of the electrode material is considerably reduced, thus attaining a longer service life even after a relative large number of short circuits. Moreover, the arc voltage of the electric arc is raised due to the enlarged distance of a connecting channel from the reachable conductive surface of an electrode. This brings about an improved current-limiting behavior of the current-limiting device and results, moreover, in a lower loading of the current-limiting device and of the electric circuit to be protected. During nominal operation, the current in the liquid metal is forced by the non-conductive ceramic disks to flow around them. In this manner, the current distribution in the electrodes is homogenized to a considerable degree as a result of which locally heated regions in the electrodes are obviated which, in turn, has a beneficial effect on the material stability thereof.
Advantageous are ceramic disks on the basis of boron nitride, silicon carbide, silicon nitride or aluminum oxide, it being possible for the ceramic disks to be adhesively bonded or soldered to the inner surfaces in an expedient manner, or to be inserted in a suitable way.
A further refinement of the present invention consists in providing the inner surfaces of the electrodes with a conductive material layer which is diffusion-resistant with respect to the liquid metal. In this manner, in addition, the diffusion and corrosion of the inner electrode surfaces which are wetted by the liquid metal is effectively coped with, resulting in a considerable increase of the surface resistivity of the electrodes and, consequently, of the stability and service life of the current-limiting device. Expediently, the material layer is composed of one of the proposed transitional metals or alloys thereof, it being possible for the material layer to be advantageously designed as a small plate which is mounted on the inner surfaces of the electrodes, for example, by adhesive bonding or soldering, or which is inserted flush with the inner surfaces of the electrodes, or as a metal layer which is applied to the inner surfaces of the electrodes by electroplating, vapor deposition or friction welding. Underneath a non-conductive ceramic disk, the material layer can either be completely or partially continued or also be cut out.
It is advantageous to use a gallium alloy as the liquid metal. Especially GaInSn alloys are easy to handle because of their physiological harmlessness. An alloy of 660 parts by weight of gallium, 205 parts by weight of indium, and 135 parts by weight of tin is liquid from 10xc2x0 C. to 2000xc2x0 C. at normal pressure and possesses sufficient electrical conductivity.